Note Ueshiba in the 1935 Asahi News film concluding techniques at various points with (somewhat) similar hand-up and hand-down positions in the standing against standing demonstrations, and sometimes in the seated v. standing demos (Ueshiba when kneeling sometimes slaps the mat/earth with his down hand when concluding a technique):

It could be a stylistic flourish, or something relevant to the performance of the technique (or both). Zanshin has been mentioned in connection with this finishing position. Is this finishing position some kind of zanshin of the body, that is, serving to reorient Ueshiba's body after a technique?

Rebalancing of yin-yang (in-yo) in the body after a technique, or a reflection of solo training in a body well-conditioned internally? Viz. Akuzawa practicing shintaijiku:

If Ueshiba was working on aiki, then we could guess that he was working on in-yo, or yin-yang, or contradictory opposites. So, his heaven-earth would be comparable to that.

I could theorize that when Ueshiba was saying things like being the bridge between heaven and earth, he was using this pose. One hand to heaven, one hand to earth, and his core body between as the bridge. Or, maybe even his upper body (chest and waist) was heaven while his lower body (hips and legs) was earth, and his hara (that central area between upper rib cage and pelvic bowl) was the bridge. Or his upper intent really was as far up as heaven, his lower intent really was at the core of Earth, and his spirit was in between being the bridge that other physical bodies came into contact with.

One could then add in that, maybe, Ueshiba was adding opening and closing to his body. Hand up opens, hand down closes.

And then, maybe, Ueshiba could have added spirals to it all so that whenever anyone touched him, they encountered his spirals that he used to effectively move people where he wanted them to go.

One could guess that perhaps that pose is the final physical outward appearance of everything that he'd already done internally as theorized above.